A polyester film is excellent in strength, dimensional stability and chemical resistance and, therefore, the film is used for optical purposes. The polyester film is especially useful for display devices such as LCDs, CRTs, PDPs and ELs.
In the field of display devices, images have recently been displayed more and more precise and reproduced in color. Accordingly, there has arisen a strong demand for a laminated film for optical use having a uniform thickness, transparency, lubricity, and colorlessness of the color tone.
A polyester film by itself lacks lubricity and is difficult to be handled. Therefore, inorganic particles such as silica, calcium carbonate, kaolin etc., or organic particles such as silicone, cross-linked polystyrene, etc., are generally mixed, as lubricants, into the polyester film to form minute projections on the surface of the film to improve the lubricity.
To improve the lubricity by this method, however, it is necessary to be mixed with at least several hundreds ppm of inorganic or organic particles. Due to these particles light is scattered to cause degradation of the transparency of the polyester film. In addition, these particles help to decompose the polyester whereby the polyester film is likely colored yellow.
To maintain the transparency of the polyester film, there have been proposed to form a lubricity layer on the surface of the polyester film without mixing the inorganic or organic particles into the film, to balance the transparency and lubricity.
Even by this method, however, it is difficult to uniformly stretch the polyester film to produce a biaxially stretched polyester film and, thus, it was difficult to obtain a polyester film having a uniform thickness and orientation.